SAN FRANCISCO--Forget the clever mnemonics and untie that string around your finger. If you really need to learn something, get a good night's sleep. Researchers have found that two types of sleep work together to help the brain absorb experiences. The finding, presented here on 5 April at the Society for Cognitive Neuroscience's annual meeting, provides new evidence that the more you dream at night, the better your brain learns.

Everyone knows that the brain works while the body snoozes--but just what the brain is up to is mostly a matter of speculation for scientists. In 1994, an Israeli team discovered that people learn a difficult visual skill better if they sleep between testing sessions (Science, 29 July 1994, p. 679). In the test, subjects briefly see a small row of symbols in their peripheral vision and must judge whether the symbols are aligned horizontally or vertically. After an hour of training, people can answer correctly even if they have seen the target for less than a tenth of a second; if they take the test again after a full night's sleep, they're even faster. Cognitive neuroscientist Robert Stickgold and his colleagues at Harvard Medical School in Boston wanted to know how much--and what kind--of sleep was necessary for this unconscious learning.

Stickgold's group monitored the sleep patterns of 14 student volunteers on the night after they learned the perceptual discrimination task--making sure each got 8 hours of sleep. Subjects who experienced no slow wave sleep (SWS)--the deepest sleep--during the first quarter of the night did not improve their performance on the perceptual task the next day. Nor did those without rapid eye movement (REM) sleep--in which dreaming occurs--during the last quarter of the night. In contrast, those with the most early SWS and late REM sleep improved the most when tested the next day. The total amount of time of early SWS and late REM predicted 80% of the variation in performance when subjects were retested. Unfortunately, there doesn't appear to be a way to control how much REM sleep or SWS a person gets at night.

"This is a quite startling finding," says Stickgold, who says he was amazed to find that two distinct periods of REM sleep and SWS appear to correspond so closely to learning. "It's a really important finding showing that sleep and learning are related," adds psychologist Carlyle Smith of Trent University in Peterborough, Ontario, Canada. While working with rats, Smith has found similar "windows of learning" that occur during REM sleep. So think twice before setting that alarm clock too early, Stickgold says: You may miss one of the most important times for learning yesterday's lessons.